A global positioning system (GPS) has been widely employed in many applications. For example, GPS can be used in vehicle navigation systems, personal locaters, aviation and the like. By using GPS together with an electronic map (E-map), a navigation system can precisely locate its carrier such as a vehicle or a hiker and program a proper route for further advancement or facilitate searching and rescuing tasks.
FIG. 1 is a schematic diagram illustrating a typical global positioning system. A positioning device 10 receives GPS signals from three or more GPS satellites 111˜11n orbiting the earth 12 and determines a position of the carrier carrying the positioning device 10 in a two-dimensional or three-dimensional coordinate system according to the GPS signals by way of triangulation.
When the positioning device 10 is turned on, previously stored almanac and ephemeris data, current time and last-identified location are checked. The almanac and ephemeris data provides estimated information of currently available satellites and locations of these satellites relative to current time and the last-identified location. The positioning device 10 then locks the currently available satellites instead of inefficiently searching satellites in the sky. If the almanac and ephemeris data are out of date or lost, the searching of GPS satellites cannot be performed smoothly. Therefore, for rapidly locating available satellites and precisely acquiring GPS signals, the almanac and ephemeris data stored in the positioning device 10 should be frequently updated.
Alternatively, in Assisted GPS (or AGPS), the almanac and ephemeris data and the temporal and spatial data are received from a base station of a mobile-phone network system. It is advantageous in reducing the positioning time and extending the covering range indoors.
FIG. 2 is a schematic functional block diagram illustrating a typical Assisted GPS. In the Assisted GPS, an A-GPS server 21 is built in a mobile-phone network 20 at such a proper position that a satellite signal receiver 210 of the AGPS server 21 can receive high-quality GPS signals from the GPS satellites 221˜22n. When a mobile phone 29 with the AGPS positioning function validly links to a base station 201 of the mobile-phone network 20, the AGPS server 21 identifies the Cell ID of the base station 201, finds an area where the mobile phone 29 is located according to the Cell ID and associated tables of the base station 201, and then transmits almanac and ephemeris data corresponding to the satellites in the area to the mobile phone 29 via the mobile-phone network 20. In comparison with conventional global positioning systems, the Assisted GPS accelerates the positioning operation and improves the positioning precision. Furthermore, the precise position information of the mobile phone 29 can be fed back to the AGPS server 21 via the mobile-phone network 20, thereby achieving a location-based service (LBS).
Since the AGPS server 21 cannot execute the positioning function without the aid of the mobile-phone network 20 for offering associated services of the Assisted GPS, it is critical to look for support from the mobile-phone service operator of the mobile phone network 20. Once the mobile-phone network 20 covers a variety of mobile-phone systems, e.g. the mobile-phone subscriber is roaming in a foreign region beyond the coverage of the contract operator, the above mechanism does not work. The highly dependence of the conventional AGPS on a mobile-phone network renders the entire system uncertain.